Traditionally, a fuel cell system capable of highly efficient and small-scale power generation easily constructs a system for utilizing a heat energy generated during power generation and achieves high energy utilization efficiency. Therefore, the fuel cell system is suitably used as a distributed power generation system.
The fuel cell system comprises a fuel cell which is a base portion of a power generation portion. As the fuel cell, for example, a polymer electrolyte fuel cell or a phosphoric acid fuel cell is typically employed. In these fuel cell systems, a hydrogen-rich gas containing plenty of hydrogen (hereinafter referred to as a fuel gas) and air (hereinafter referred to as an oxidizing gas) are used to generate power. To this end, the fuel cell system is equipped with a fuel processor configured to generate the fuel gas required for power generation. In the fuel processor, for example, a natural gas supplied from a natural gas supply means or the like goes through a steam reforming reaction and is converted into hydrogen, thus generating the fuel gas containing plenty of hydrogen. In this case, a reaction space in which the steam reforming reaction takes place in the fuel processor is heated up to and kept at a predetermined temperature by heat resulting from combustion of, for example, the natural gas.
In an operation of the conventional power generation system including the fuel cell system, in order to inhibit wasteful consumption of the fuel gas or the like used in power generation, it is desirable to properly control the supply amount of the fuel gas or the like to the fuel cell or the like according to a power consumption of a power load such as electronic equipment (hereinafter referred to as a power load) connected to the power generation system. In other words, in the fuel cell system, in order to inhibit wasteful consumption of the natural gas or the like used to generate the fuel gas, it is desirable to properly control the amount of the natural gas supplied to a fuel reformer according to the power consumption of the power load. Accordingly, there has been disclosed a power generation system in which power resulting from power generation and commercial power are used together in a time period in which the power consumption of the power load connected to the power generation system is large in amount, while the power generation is stopped and only the commercial power is supplied in a time period in which the power consumption of the power load is small in amount (e.g., Japanese Laid-Open Patent Application Publication No. 2000-299116). In addition, there has been disclosed a power generation system in which when the power consumption of the power load is not less than a predetermined threshold, an output power is controlled according to a variation in the power consumption of the power load detected by a load power detecting means, while when not more than the predetermined threshold, the power generation operation is stopped (e.g., Laid-Open Patent Application Publication No. 2002-352834). According to these disclosures, since the consumption amount of the feed material such as the natural gas, required for power generation is properly controlled according to the power consumption of the power load, it is possible to construct a suitable power generation system with higher energy utilization efficiency.
Hereinafter, a construction and operation patterns of the conventional fuel cell system will be described with reference to the drawings.
FIG. 6 is a block diagram schematically showing a construction of the conventional fuel cell system.
As shown in FIG. 6, a conventional fuel cell system 100 comprises a fuel cell 100a configured to generate power using a fuel gas and an oxidizing gas, an output control means 100b configured to control an output power of the fuel cell 100a and to control start-up and stop of a power generation operation of the fuel cell 100a, a load power detecting means 100c configured to detect a power consumption of a power load 100e described later and to output a control signal necessary for, for example, the output control means 100b to control the output power of the fuel cell 100a, and a storage battery 100d configured to store excess output power. The storage battery 100d is electrically connected to a connecting portion between the output control means 100b and the load power detecting means 100c. In addition, a commercial power 100f is connected to the connecting portion. The power load 100e, such as electronic equipment, is configured to consume the power output from the fuel cell system 100 and is connected to the load power detecting means 100c. 
In the conventional fuel cell system 100 of FIG. 6, a fuel gas generated in a fuel gas generation means such as a fuel processor which is not shown in FIG. 6 and an oxidizing gas are supplied to the fuel cell 100a. Using the fuel gas and the oxidizing gas, the fuel cell 100a generates power. The output power resulting from power generation in the fuel cell 100a is supplied to the power load 100e through the output control means 100b and the load power detecting means 100c. The power load 100e consumes the power supplied from the fuel cell system 100. At this time, excess output power is stored in the storage battery 100d. When the output power of the fuel cell 100a is insufficient for the power consumption of the power load 100e, the commercial power 100f makes up for the deficiency.
An example of an operation pattern during a day regarding the power generation operation in the conventional fuel cell system will be described in detail.
FIG. 7 is a view schematically showing an operation pattern during a day of the conventional fuel cell system. In FIG. 7, an ordinate axis indicates a power axis and an abscissa axis indicates a time axis.
In FIG. 7, a curve 111 indicates a time lapse variation in the power consumption of the power load 100e, and a curve 112 indicates a time lapse variation in the output power of the fuel cell 100a. In FIG. 7, a maximum output power W1c indicates a maximum value of the power that is capable of being output from the fuel cell 100a, and a minimum output power W1d indicates a minimum value of the power that is capable of being output from the fuel cell 100a. 
As illustrated by the curve 111 of FIG. 7, typically, the power consumption at general home in a first time period 101a from about 0:00 at midnight until about 5:00 in early morning is small in amount, but is large in amount in a second time period 101b that elapses from when users wake up until about 13:00 when household tasks may be finished. The power consumption is small in amount in a third time period 101c from about 13:00 until about 17:00 because of a decrease in the number of power loads 100e that are in operation, but is large in amount in a fourth time period 101d from about 17:00 until about 23:00 because of an increase in the number of the power loads 100e that are in operation. The power consumption in a fifth time period 101e after the users go to sleep is small in amount as in the power consumption in the first time period 101a. 
The fuel cell 100a in the conventional fuel cell system 100 outputs power as indicated by the curve 112 of FIG. 7 in response to the variation in power consumption during a day. Specifically, when the load power detecting means 100c of the fuel cell system 100 detects that the power consumption of the power load 100e is above an operation start power threshold W1a which is a preset threshold at which the fuel cell 100a starts a power generation operation for a predetermined time period T1a or more in the first time period 101a in FIG. 7, the power generation operation of the fuel cell 100a is started-up (first start-up). The fuel cell 100a starts to output power as indicated by the curve 112 after an operation preparation time period Ts in which, for example, a fuel gas is generated in a fuel processor or the like. When the power output of the fuel cell 100a becomes substantially equal to the power consumption of the power load 100e in the second time period 101b, the output control means 100b controls the output power of the fuel cell 100a between the maximum output power W1c and the minimum output power W1d according to the variation in the power consumption of the power load 100e which is detected by the load power detecting means 100c. At this time, if the power consumption of the power load 100e is above the output power of the fuel cell 100a, the commercial power 100f makes up for the deficiency. As illustrated by the third time period 101c, when the power consumption of the power load 100e is below an operation stop power threshold W1b for a predetermined time period T1b or more, the output control means 100b stops the power generation operation of the fuel cell 100a. At this time, the operation for generating the fuel gas in the fuel processor or the like is stopped. In a stopped state of the power generation operation in the fuel cell 100a, the commercial power 100f supplies the power to the power load 100e. 
As illustrated by the third time period 101c, when the load power detecting means 100c of the fuel cell system 100 detects that the power consumption of the power load 100e is above the operation start power threshold W1a for the predetermined time period T1a or more, the power generation operation of the fuel cell 100a is re-started-up (second start-up). As in the first start-up, the fuel cell 100a re-starts to output the power as indicated by the curve 112 after the operation preparation time period Ts. As in the second time period 101b, as illustrated by the fourth time period 101d, the output control means 100b controls the output power of the fuel cell 100a between the maximum output power W1c and the minimum output power W1d according to the variation in the power consumption of the power load 100e which is detected by the load power detecting means 100c. 
As illustrated by the fifth time period 101e, when the power consumption of the power load 100e is below the operation stop power threshold W1b for the predetermined time period T1b or more again, the output control means 100b stops the power generation operation of the fuel cell 100a again. As in the third time period 101c, at this time, the operation of the fuel processor or the like is stopped. In this case, the commercial power 100f supplies the power to the power load 100e. 
As should be appreciated, in the conventional fuel cell system 100, the output power of the fuel cell 100a is controlled according to the variation in the power consumption of the power load 100e. When the power consumption of the power load 100e transitions from a large amount state in, for example, the second time period 101b, to a small amount state in, for example, the third time period 101c, and the power consumption of the operation stop power threshold W1b or less continues for the predetermined time period T1b or more, the power generation operation of the fuel cell 100a and the operation of the fuel processor or the like are stopped.
However, in the above described conventional fuel cell system 100, when the power generation operation of the operation pattern illustrated in FIG. 7 is performed, wasteful consumption of the natural gas or the like occurs because of the second start-up. More specifically, in the conventional fuel cell system 100, the start-up of the fuel cell 100a, the fuel processor or the like is performed twice during a day, like the first start-up and the second start-up. It is important that, in stopping the power generation operation of the fuel cell 100a during a relatively long time period, for example, in a range from the fifth time period 101e to the first time period 101a, the operations of the fuel cell 100a, the fuel processor and the like be stopped in order to increase energy utilization efficiency. However, when the power generation operation of the fuel cell 100a is stopped during a relatively short time period as illustrated by the third time period 101c, the energy required to start-up the fuel cell 100a, the fuel processor, or the like is more than the energy consumed by continuing the power generation operation of the fuel cell 100a. In other words, when the power consumption of the power load 100e is less for a relatively short time period, a total energy utilization efficiency increases by continuing the power generation operation of the fuel cell 100a. According to this, since the natural gas or the like is wastefully consumed to start-up the fuel cell 100a, the fuel processor, or the like, which is believed to be unnecessary, the total energy utilization efficiency decreases. As compared to other power generation systems, for example, an engine power generation system or the like, such unnecessary start-up operation increases wasteful power consumption in the fuel cell system that reforms a feed material such as the natural gas (city gas) to generate the fuel gas, because of its longer time period of the start-up operation, and as a result, the total energy utilization efficiency decreases.